Oscillators are well known in the art and important in providing clock signals in digital logic circuits. Clock generation for semiconductor devices can take many forms including ring oscillators, crystal controlled oscillators, external clock devices, Phase Locked Loops (PLL) on a semiconductor device, Delay Locked Loops (DLL) on a semiconductor device, and various combinations of the above. Crystal controlled oscillators are generally useful for precisely creating a desired frequency, but cannot directly produce the very high frequencies required in a test chip designed to evaluate high performance circuitry. Similarly, external clock generators vary greatly in precision and frequency, but they are generally designed to maintain a precise fixed frequency and create global clocks for distribution within a system. As a result, external clock generators tend to be expensive.
Generally high frequency clocks on a system board are generated by expensive clock generators and maintaining a clean clock signal at high frequencies is problematic. To overcome this problem, many semiconductor devices use PLL's, which create internal clocks at higher frequencies, generally multiples of a lower frequency external reference clock. To be accurate, yet flexible enough to generate a large variety of frequencies, PLL's can be difficult to design. PLL's generally require analog circuit design techniques, and may still not provide the flexibility required for a test chip where varying the frequency of the clock is valuable in analyzing various performance parameters of a test chip.
DLL's may also be used to create clock multiples for an internal clock signal from a lower frequency clock reference. Some DLL's do not require analog circuitry but generally have the same problems of design complexity and lack of flexibility as a PLL solution when used for a test chip. However, DLLs are also often used to create phase shifts in an internal clock on a semiconductor device relative to a reference clock. When used as a phase-shifting device, DLL's may be quite useful in a test chip.
For a test chip, there is a need for a low cost high frequency oscillator solution that is flexible both in creating a desired frequency and in the ability to easily modify the created frequency.